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Bob McDavitt's ideas for sailing weather around the South pacific

07 May 2017

Bob Blog 7 May 2017

WEATHERGRAM
YOTREPS

Compiled Sun 7 May 2017
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

Tropical Cyclone DONNA
DONNA may have formed a few days "out of season". However, this should be no great surprise as the sea temperatures all around the tropical Pacific are warmer than normal and that's the trigger.
Sea Surface temperature anomalies may be seen at www.ospo.noaa.gov/data/sst/anomaly/2017/anomp.5.4.2017.gif
These warmer than normal seas are already breeding tropical depressions in the NE Pacific, and their "Cyclone Season" is meant to start on 15 May, but there is already an interesting collection of possible depressions off the Honduras west coast this week.

To see how the annual weather cycle and the seasons are working out, check out the average isobar maps for April at
www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html

Back in March, the subtropical ridge was near latitudes 30 in both N and S hemispheres. Then, in April, that ridge noticeable shifted NORTHWARDS, especially over the mainlands of Australia and South America, and in the northern Hemisphere it has weakened over the mainlands of Eurasia and North America.

The anomaly map shows that that the troughy area that was over Australia in March has been replaced by a ridgey area--- it seems to have weakened and shifted to the NZ area. This is consistent with the Tasman Tempest and Cyclone Cook events that brought NZ record April rainfall.

The Last 30 days of rainfall, and its anomaly, as seen at TRMM
trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html
clearly shows that the wetter than normal conditions over NZ. This also shows Vanuatu had a wetter than normal April.

The Tropics
TC DONNA reached Cat 4 early today and is now Cat 3 and continuing to curve South then south-southeast then southeast reaching the Iles des Pins area by around 5pm local time Wednesday. By then it should have weakened to Cat 2.

Its latest official track for DONNA is available from www.met.gov.fj/aifs_prods/65661.html

A trough is crossing that Tasman Sea and moving onto New Zealand later this week. This trough should capture the remains of DONNA when it exits from the tropics on Thursday and help push them south-eastwards off to the east of NZ on Friday/Saturday. DONNA is likely to lose strength when it leaves the tropics because it will encounter a Jetstream and get decapitated. Cyclone COOK, by way of comparison, was able to get between two jetstreams and thereby deepen as it approached NZ.

There is a small tropical depression over the Tonga area tonight. It is expected to fade as it moves off to the north of Fiji over next few days.

The rain from cyclone DONNA in the past few days is the highlight of this week's rain map at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif
especially when compared with last week.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
SPCZ is expected to remain strong in a zone from Tonga to Southern Cooks.

Subtropical ridge (STR)
High over northern New Zealand tonight is travelling off to the east.
Next HIGH from the Australian bight is expected to travel east across Tasmania on Friday 12 May, and then across the Tasman Sea next week reaching northern New Zealand by Friday 19 May.

Australia to New Caledonia:
The GO EAST RALLY is rearing to go soon.
SEE www.downunderrally.com/about-go-east
The HIGHS crossing the Tasman Sea are producing too much of a Southeast wind on this route for a good sail to Noumea. The Low that is expected to be crossing the Tasman Sea this week (with a trough that captures the remains of DONNA) is too far south to turn the winds southerly long enough for a nice voyage to Noumea. SO, may as well stay put this week.
And during next week there is another HIGH, maintaining more SE winds along the route until at least Thu 18 May. Perhaps soon after that there should be a favourable swing of wind. More about this next week.

Departing NZ to the north for the tropics:
Trough is moving onto New Zealand from the Tasman Sea, and crossing the North Island on Thursday. Because it is preceded by increasing NE winds, may as well stay put until this trough has gone. AT this stage, a departure on Friday 12 or Sat 13 May is looking good.

New Zealand to the east (Tahiti)
After the Thursday trough we may need to wait a day or so for the southerly winds that follow that trough to ease. Then around Sat 13 or Sun 14 May there should be an OK opportunity for departure.

Mexico to Marquesas:
If a tropical depression does deepen off the Honduras coast next few days then it should go north and make landfall near the Guatemala/Mexico border by Sun 14 May, and thus not affect anyone departing from La Paz area. Keep an eye out just in case.
Off NW Mexico, there should be a useful north to NW flow for departure anytime this week, but it may fade next week.
The ITCZ is likely to be between 7N and 2N, and light winds between 7N and 2N and from 1S to 3S. Different from last week, and perhaps better sailing winds if you aim for a waypoint EAST of the direct path this week near maybe EQ 123W.

Panama to Galapagos
Southerly winds off Panama are light until Monday then expected to become moderate until wed 17 May.
SO, Sunday/Monday offer the best possible departure conditions this week.
At 7N we may be deflected to west southwest by the S/SSE winds - sadly that's into the main convective clouds with squalls.

Galapagos to Marquesas
There is still a zone of light winds about and wets of Galapagos.
To avoid these, go to 5N 93W and then can go direct.
Maybe a convergence zone with squalls near 9S 120W, perhaps.

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